1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copier which adopts an electrophotographic system.
2. Related Background Art
Conventionally, an image forming apparatus adopting an electrophotographic system comprises: a latent image bearing body for bearing a latent image; developing means for visualizing the latent image as a developer image by supplying a developer to the latent image bearing body; transferring means for transferring the developer image onto a recording medium; and fixing means for fixing the developer image on the recording medium by heating and pressurizing the recording medium which bears the developer image.
Further, in such an image forming apparatus, there has been known and put into practice an image forming apparatus comprising residual charge elimination means which charge-eliminates the recording medium by applying a bias from a power source to the recording medium after the transfer of the developer image to the recording medium by the transferring means.
For example, in such an image forming apparatus, as shown in FIG. 7, as a method of separating a paper 114, which is the recording medium after the toner image which is the developer image was transferred, from a photosensitive drum 103 which is the latent image bearing body, a method is generally adapted wherein, by applying a bias voltage of the polarity opposite to the bias applied (hereinafter, referred to as a residual charge elimination bias) to the transferring roller 107 which is transferring means, a charge on the paper 114 is charge-eliminated and an absorption force between the photosensitive drum 103 and the paper 114 is weakened.
A residual charge elimination needle 120 has a current-limiting resistor 121 having a resistance of 5 xcexa9 to 50 xcexa9 connected to a power source 122 in series to prevent a current leakage due to a high voltage. In the present example, in order to prevent the bias applied to the transferring roller 107 from leaking to the residual charge elimination needle 120, a residual charge elimination needle holder 120a for shielding between the transferring roller 107 and the residual charge elimination needle 120 is installed. Note that, in general, the above described method of separating the paper 114 from the photosensitive drum 103 by the residual charge elimination needle 120 is used in combination with a method of abutting a separating claw (not shown) against the photosensitive drum 103 and forcing the paper to be stripped off after transfer in order to enhance separability of a thin paper from the photosensitive drum 103.
In a fixing apparatus 117 provided for such an image forming apparatus, as shown in FIG. 7, the paper 114 which bears an unfixed toner image T is passed through a nip formed by a roller heated from the inside by a heater 133 which is heating means, thermal fixing means 131 which is a film-shaped fixing member and a pressure roller 132 which is a pressing body and pressure-welded to the thermal fixing means 131, and heated and pressurized so that the unfixed toner image T is fixed on the paper 114.
In recent years, in order to solve an offset problem in such a fixing apparatus 117, there have been on the increase apparatuses having such a constitution that, on the surfaces of the thermal fixing means 131 and the pressure roller 132, a potential difference is induced in such a direction as to press an unfixed toner image on the paper 114 against the paper 114, the offset toward the thermal fixing means 131.
For example, as for the fixing apparatus of the above described constitution, there are such apparatuses available wherein, by applying a bias (if a negative toner, xe2x88x92100 V to xe2x88x922000 V) of the same polarity as the toner (unfixed toner) to the metal core of the fixing roller, a potential which repels the toner is induced on the surface, while, on the other hand, the pressure roller disperses an electrically conductive agent on a surface layer fluororesin layer and an elastic layer to make it as a medium resistive roller (having a surface resistance of 107 xcexa9 to 1012 xcexa9) and, by connecting a diode to the metal core, the potential difference with the fixing roller is maintained.
Or there are such apparatuses available, wherein, as shown in FIG. 7, the thermal fixing means 131 (for example, the fixing roller metal core) is grounded (of course, it may be applied with a bias of the same polarity as the toner), and the surface resistance of the pressure roller 132 is made a low resistive not more than 106 xcexa9, and an electrode of an electrically conductive brush 134 and the like applied with a bias (hereinafter, referred to as a pressure bias) of the polarity reverse to the toner by a power source 136 is brought into contact with the surface of the pressure roller 132, so that the potential difference with the thermal fixing means 131 is maintained.
This apparatus comprises a current-limiting resistor 135 of 100 Mxcexa9 to 1000 Mxcexa9 which controls a large electric current so as not to flow even if the pressure bias of high voltage continues to be applied and, compared to the current-limiting resistor 121 of the residual charge elimination bias, a high value resistor is used for the current-limiting resistor 135.
In this way, by generating a potential difference in such a direction as to press the toner on the paper, there is no more to coat a surface lubricant such as a silicone oil and the like on the surface of the thermal fixing means as in the past or to make a cleaning member abut against the thermal fixing means or the pressure roller.
As a result, there are no more accidents such as an oil leakage and the like and a user""s labor to periodically replace the cleaning member is eliminated.
FIG. 8 is a chart showing a timing of the residual charge elimination bias at the time when the paper 114 passes through on the residual charge elimination needle 120 with the position of the residual charge elimination needle 120 as a reference. Note that, in FIG. 8, an arrow mark is a top end of the paper 114.
The residual charge elimination bias adequately eliminates the charge on the paper and applies a bias of xe2x88x920.5 kV to xe2x88x921.5 kV (hereinafter, referred to as a low level) from the top end of the paper to the halfway of the image so that the paper does not stick and pile by a guide member installed between the transferring portion and the fixing portion. While, at the paper rear end, it is preferable to strongly charge-eliminate the paper by a bias of xe2x88x922 kV to xe2x88x923 kV (hereinafter, referred to as high level) so that the paper does not jump up to allow the image to rub against the base of a cartridge.
By the way, usually, in the miniaturized apparatus, the paper is arranged so as to stretch across the transferring portion and the fixing portion and, when such an image forming apparatus as described above is sued under high humidity, because the paper is humidified under high humidifying circumstances, the potential of the pressure roller is lowered by a strong bias applied to the residual charge elimination needle so that the paper rear end does not jump up after the toner image is transferred on the paper. For this reason, there were some cases where a force to retain the unfixed toner image on the paper runs out and black spots phenomenon on a paper (hereinafter merely referred to as black spots) occurs at the fixing portion.
That is, in the case where the fixing apparatus is an apparatus wherein the surface resistance of the pressure roller is set not more than 106 xcexa9 and the bias of the polarity reverse to the toner is applied to the pressure roller, when a strong bias of the residual charge elimination needle is applied to the paper rear end as shown in FIG. 9, the potential of the humidified paper is lowered (B of FIG. 9). Incidentally, if the paper rear end passes through the residual charge elimination needle, a charge is newly supplied from the pressure roller and the potential of the paper becomes the same as the potential applied to the pressure roller.
Further, A, C of FIG. 9 show the fluctuations of the pressure roller potential at the moment when the paper reaches the fixing nip and at the moment when the paper leaves from the fixing nip, respectively.
In this way, when the top end of the paper whose charge is eliminated is brought into contact with the roller, or when the potential of the pressure roller is fluctuated by a separating discharge of the paper rear end, the black spots occur sometimes at a portion in which the potential of the roller was lowered.
It is an object of the present invention to control the potential fluctuation of the fixing member and provide an image forming apparatus for preventing black spots of toner.
It is another object of the present invention to provide image forming apparatus, comprising an image bearing body; transferring means for transferring a toner image on said image bearing body to a recording medium; fixing means for fixing an unfixed toner image transferred on a recording medium by said transferring means on the recording medium; voltage applying means for applying a voltage to a second fixing member which is not brought into contact with an unfixed toner image of said fixing means; and residual charge elimination means for charge-eliminating a recording medium before being fixed by said fixing means, in which said fixing means comprises first and second fixing members for holding and conveying the recording medium, at least a recording medium of the maximum size exists by extending from said residual charge elimination means to said fixing means, said voltage applying means comprises a first power source and a first resistor connected to said first power source in series, a voltage is applied to said second fixing member by said first power source through said first resistor, said residual charge elimination means comprises a second power source and a second resistor connected to said second power source in series, a voltage is applied to the recording medium by said second power source through said second resistor, and when the voltage value of said first power source is taken as V0, the resistance value of said first resistor as R0, the voltage value of said second power source as V1, and the resistance value of said second resistor as R1, there is a relation represented by V0xc2x7R1 greater than V1xc2x7R0.
It is another objected of the present invention to provide an image forming apparatus, comprising an image bearing body; transferring means for transferring a toner image on said image bearing body onto a recording medium; fixing means for fixing an unfixed toner image transferred on a recording medium by said transferring means on the recording medium; voltage applying means for applying a voltage to a second fixing member which is not brought into contact with an unfixed toner image of said fixing means; and residual charge elimination means for charge-eliminating a recording medium before being fixed by said fixing means, in which said fixing means comprises first and second fixing members for holding and conveying the recording medium, at least the recording medium of the maximum size exists by extending from said residual charge elimination means to said fixing means, said voltage applying means comprises a power source, and a resistor and capacitor which are connected to said power source in series, and a voltage between said resistor and said capacitor is applied to said second fixing member.
It is another objected of the present invention to provide an image forming apparatus, comprising an image bearing body; transferring means for transferring a toner image on said image bearing body onto a recording medium; fixing means for fixing an unfixed toner image transferred on a recording medium by said transferring means on the recording medium; voltage applying means for applying a voltage to a second fixing member which is not brought into contact with an unfixed toner image of said fixing means; residual charge elimination means for charge-eliminating a recording medium before being fixed by said fixing means; and an electrically conductive member brought into contact with the recording medium between said residual charge elimination means and said fixing means, in which said fixing means comprises first and second fixing members for holding and conveying the recording medium by pressure-contacting portions mutually pressure-contacted, at least the recording medium of the maximum size exists by extending from said residual charge elimination means to said fixing means, said voltage applying means stops applying the voltage to said second fixing member when the rear end of the recording medium reaches the pressure welding portion of said fixing means, and said electrically conductive member is grounded through the capacitor.
Still another object of the present invention will be evident from the following description.